1. Field of the Invention
The present invention relates to an image signal processing apparatus and method, and more particularly, to a dynamic range magnifying apparatus and method based on a histogram equalization technique.
2. Description of the Related Art
An image generated by a video image input device such as a camcorder includes various information on an object. However, information on illuminance and color is frequently biased or distorted in a photographed image. The bias or distortion can be attributable to the object of the photograph, the performance of the photographing apparatus, the photography conditions, etc. In many cases, to compensate for these effects, an image illuminance contrast and a dynamic range must be selectively amplified by special preprocessing.
A conventional method for improving images is a homomorphic filtering method based on a transform operation. FIG. 1 is a block diagram of a dynamic range magnifying apparatus using the homomorphic filtering method. Referring to FIG. 1, an input image signal is divided into an illuminance component and a reflectance component by a logarithmic transformer 10, and only the reflectance component is amplified by a high pass filter 12. An amplified signal is exponentially transformed by an exponential transformer 14, and the result is then output. A drawback of this method is that a large amount of calculation is required.
Alternatively, the image can be improved by a histogram equalization method based on a point operation. This histogram equalization method magnifies a dynamic range by normalizing an illuminance distribution of an image such that it is uniform. FIG. 2 is a block diagram of a typical dynamic range magnifying apparatus using the histogram equalization method. Referring to FIG. 2, an analog-to-digital (A/D) converter 20 converts an input image signal into a digital image. A histogram calculator 22 calculates the histogram of the digital image, e.g., a probability density function. A histogram accumulator 24 calculates the histogram to obtain an accumulation density function. The calculated accumulation density function is stored in a look-up table (LUT) 26 and used to convert the digital image. The digital image signal is reconverted into an analog signal by a digital-to-analog (D/A) converter 28, and the result is then output.
In such a system, when frequency is equalized as described above, this method suffers from some drawbacks. For example, quantized noise is amplified with a great change in the contrast of an illuminance band having high frequency. Also, the contrast is relatively lowered in an image portion of an illuminance signal having low frequency. In order to solve this problem, a linear histogram modification method has been proposed to improve the contrast while entirely reducing the effect of the histogram equalization by dividing a histogram into a plurality of bands and using a separate transform function for each band. However, this method requires a complicated circuit and cannot improve both the contrast and noise characteristics.
Another alternative for magnifying dynamic range can be a method of increasing the sensibility for an image by subdividing an image quantization step. In an actual camcorder, an image is usually quantized to 8 to 10 bits, thus providing an image sensibility of about 45 to 50 dB. On the other hand, it is known that human eyes have a sensibility of about 120 dB. Also, humans have nerve cells that are sensitive to bright objects and nerve cells that are sensitive to relatively dark objects. Thus, image recognition occurs in the nerve system in which sensitivity selectively differs according to a color temperature. U.S. Pat. No. 5,144,442 discloses a method for producing an image which has a wider dynamic range and is close to a human vision, by synthesizing an input image with several different exposures, in consideration of the above-described visual characteristics. A drawback of this method is that a plurality of input channels are required when applied to an actual camera, and a complicate camera system is thus provided. Also, image contrast may be deteriorated.